Rotating screens are known for use in many applications including in pulp and paper applications, municipal sewage treatment, meat processing, citrus fruit processing, canning, tanning, poultry processing, sugar mill processing and seafood processing.
Known rotary screening devices typically have a hollow screening cylinder journalled for rotation about a horizontal axis. Influent to be screened is internally fed to the interior of the rotating cylinder with filtrate to pass outwardly through the screening cylinder and solids and other materials in the influent which cannot pass through the screening member being retained internally with the cylinder. Rotation of the cylinder moves the solids axially along the cylinder to a discharge end.
Many known screening cylinders are made from wedge wire screen such as that described in U.S. Pat. No. 3,876,548. This screen is made up of a plurality of parallel screen bars in which each bar has an outer face and immediately adjacent side with the angles formed by a line perpendicular to the outer face and immediately adjacent the side being in the range of about 7.degree. to about 45.degree.. Such wedge wire screen has been believed to provide advantageous screening properties. Cylindrical screens made with the wedge wire device suffer the disadvantage that they are expensive and difficult to manufacture. Typically, wedge wire screens are used in minimum opening sizes of 500 microns.
Known rotary screening devices utilize a hollow cylindrical screen which is manufactured as a unitary element. The cylindrical screen is a permanent, fixed part of the screening device. This has a disadvantage that a single screening device cannot be modified so as to accept screens with different mesh sizes and thus permit the device to be utilized at different times for different filtration applications. That the cylindrical screen is a unitary element has the disadvantage that if damaged or severely clogged, either the entire device must be disassembled at substantial time and cost or use of the entire device is prevented until cleaning maintenance can be performed. For example, with a wedge wire device, damage to the wedge wire can require the entire cylinder to be replaced.
Previously known rotary screening devices have suffered the disadvantage that the screening members have not proved satisfactory to adequately remove suspended solids, notably, fat, oil and grease. In particular, in food processing industries, known cylindrical screens have had difficulty in reducing the fat, oil and grease content in products such as poultry and have had difficulty in providing an adequate biochemical oxygen demand or BOD. BOD is a measure of the quantity of oxygen utilized in the biochemical oxidation of organic matter at a specified time and at a specified temperature. New environmental requirements on maximum waste water content of solids, FOG and BOD have greatly increased filtration requirements to levels not met by existing devices.